The present invention relates to equipment and processes for thermally processing canned food products.
Conventional methods of preserving canned foods after the cans have been filled and closed require that they be subjected to heat treatment for a given period of time. This commercial process provides a sterile product with excellent keeping qualities, and the product can also have nutritive value and organoleptic (color, taste and aroma) properties which are equivalent to or even better than those of home canned products.
To reduce the amount of labor required in the process and to properly control the sterilization process, many canneries have installed continuous sterilizers, such as those made by FMC. These tall, continuous, high capacity sterilizing cookers have been built in recent years since the thinking was that faster, bigger and continuous were better. Another example is the "Hydromatic" sterilizer, which is an automatic continuous sterilizer available from Stork. It typically includes a first heat-up tower, followed by a pair of sterilizing towers, a cooling leg tower, a pair of spray-cooling towers, and a drying section tower to prevent recontamination. The cans are fed into the towers by an automatic feed and are carried through the system by a can holder chain arrangement and fed out of the system by an automatic discharge. This sterilizer can handle vegetables, meats, baby foods, corn, fruits, soups, milk, cream, milk drinks, and evaporated milks in almost any shape of today's cans.
Heavy insulation and all-weather aluminum coverings allow the Hydromatic to be placed outdoors. A centralized drive can be positioned on the top of the cooker, such as above the final drying leg tower, and the shafts and wheels thereof allow the chain to reverse its direction of travel. The chains are constructed to be sturdy and to resist the risk of breakage through corrosion from the system's steam and water. The carriers are open, acting as bucket elevators, and thus the cans can be fed in along any point of the full length or height of the preheating tower and discharged at any point between the top and the bottom of the last cooling tower. Thus, one or even several feeder lines can be connected to the cooker at any height. Special carriers can be used to handle cans of unusual shapes or sizes.
The sterilizer uses the hydrostatic principal wherein steam pressure in the sterilizing section is balanced by water columns on either side thereof. The height of the columns depends on the required steam pressure, that is on the temperature. The hydrostatic head can be adjusted through overflow valves so that the sterilizing temperature can be varied within set limits. The general concept of the cooker or sterilizer can thus be demonstrated by taking two bell jars, inverting one inside of the other and filling them with water. The inner one is then pressurized, and the water is pushed down and into the legs on either side of the space between the two bell jars. The columns of water on the outside add reverse pressure in the opposite direction and balance out at the pressure in the inside of the inner jar or steam dome.
The can carriers of the hydromatic are fixed between a pair of endless conveyor chains which traverse the entire unit and take the cans through the preheating tower, the steam tower and the several cooling towers. The average temperature in the preheating column is close to the boiling point, and the cans which are filled at, for example, 160.degree. F. are gradually heated as they pass through it. The sterilizing tower contains saturated steam and a number of up and down passages through this steam. The number of passages depends on the sterilizing time required. Since the chain speed and the water columns are variable, a wide variety of time-temperature combinations are possible. When the steam pressure drops, the water level rises and a float automatically opens a steam supply valve to admit steam and restore the pressure. After sterilizing, the cans are rapidly cooled in the cooling section by a counter current flow of cooling water. The cooling water, after being heated by the cans, is transferred to the preheating section where it is used to warm the incoming cans. The pressure and temperature drop in the first cooling leg, however, are gradual to reduce leakage in the seams of the can.
The cans can also be rotated throughout the preheating, sterilizing and cooling towers at a rate of, for example, from zero to twelve rotations per minute. The open carriers automatically pick up the cans which are waiting in rows for pickup. This open can carrier design allows for easy unloading since the cans are simply guided into a curve which causes them to slide out. The cans after discharge can be conveyed laying on their sides to a labelling machine.
Thus, cans from the filler lines are passed into the hydrostatic cooker/sterilizer for processing. The steam is introduced into the hydrostatic sterilizer at a fixed temperature, and the dwell time of the product in the sterilizer is adjusted by adjusting the conveyor chains that carry the product through the sterilizer. Different products, however, require different processing conditions in the sterilizer. Thus, for a different product it was necessary to empty the sterilizer and adjust the condition for the next product, which procedure takes an hour or two. Hence, it was not generally practical to change products during the course of an operating day since this would result in excessive down time. Instead, the practice has been to produce one product for the entire operating day, thereby limiting the number of different products manufactured per shift, and to inventory the excess. These large inventories cost major food processing companies millions of dollars per year.